A corona discharge is an electrical discharge characterized by a corona and occurring when one of two electrodes placed in a gas (i.e. a discharge electrode) has a shape causing the electric field on its surface to be significantly greater than that between the electrodes. The two electrodes are generally asymmetric. A discharge electrode, which has a low radius or high curvature, may be shaped as a sharp needlepoint or narrow wire. The passive electrode, which has a much larger radius or lower curvature, e.g. a flat plate or cylinder, is electrically grounded. The high curvature ensures a high potential gradient around the discharge electrode for the generation of localized plasma. Corona discharges are usually created in gas held at or near atmospheric pressure though in some instances they can be created in low vacuum. The discharge electrode is held at a high voltage. The corona discharge appears as a luminous glow located in space around the discharge electrode, for example, in a highly nonuniform electric field around the needle point-tip. See e.g., R. S. Sigmond and M. Goldman, “Corona discharge physics and applications” in Electrical Breakdown and Discharges in Gases, E. E. Kunhardt and L. H. Luessen, Eds. New York, Plenum (1983), pp. 1-64, Y. P. Raizer, Gas Discharge Physics, New York, Springer-Verlag (1991), J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases. London, U.K., Oxford Univ. Press (1953), L. B. Loeb, Electrical Coronas, Berkeley, Los Angeles, Calif., Univ. California Press, (1965).
The generation and the characteristics of a corona discharge are highly dependant on geometry of the discharge electrode. Electric field intensity is higher around the surface of a charged conductor or discharge electrode, which has a low geometrical radius (i.e. high curvature). Therefore, in general, the needle point-tip of the discharge electrode is made to have the smallest radius (i.e. the highest curvature) than other surface part of the electrode. If Q is the total charge stored in a needle conductor and R is its radius of the needle point-tip, the electric field intensity E at a distant r, at least approximately, is given by the following equation:E=Q/(4π ε0 r),where ε0(=8.852×10−12 F/m) is the free space permittivity and where r>R.
Therefore, as the radius R of the needle point-tip decreases, the intensity of the electric field increases around the needle point-tip. The electric field is inhomogeneous. The corona discharge is most likely to occurs around the needle point-tip of the discharge electrode.
Corona discharge may be positive or negative according to the polarity of the voltage applied to the higher curvature electrode i.e. the discharge electrode. If the discharge electrode is positive with respect to the flat electrode, the discharge is a positive corona, if negative the discharge is a negative corona. The physics of positive and negative corona are strikingly different.
A positive corona can be viewed as two concentric regions around the discharge electrode. The inner region contains ionizing electrons and positive ions, which form a plasma. In the plasma, an avalanche of electrons generates further ion/electron pairs. The outer region, which is known as the unipolar region, consists almost entirely of slow-moving massive positive ions, which migrate toward the lower curvature electrode.
In contrast, a negative corona can be divided into three radial regions around the sharp discharge electrode. In the innermost region, which is known as the ionizing plasma region, high-energy electrons inelastically collide with neutral atoms/molecules and cause avalanches, whilst outer electrons, usually of a lower energy, combine with neutral atoms/molecules to produce negative ions. In the intermediate region, which is known as the non-ionizing plasma region, the electrons combine to form negative ions, but typically have insufficient energy to cause avalanche ionization. However, the electrons but remain part of a plasma owing to the different polarities of the species present, and their ability to partake in characteristic plasma reactions. In the outermost region, which is known as the unipolar region, only a flow of negative ions and free electrons toward the positive electrode takes place. The inner two regions (i.e. the ionizing and non-ionizing plasma regions) are together known as the corona plasma. A negative corona can be sustained only in gases having electronegative molecules, which can capture free electrons.
Notably, the positive and negative coronas differ in the matter of the generation of secondary electron avalanches,. In a positive corona, the gas surrounding the plasma region generates the secondary electron avalanches with newly generated secondary electrons traveling inward. In contrast, in a negative corona, the discharge electrode itself generates the secondary electron avalanches with the newly generated secondary electrons traveling outward.
Corona discharges can occur in a wide pressure range from the low vacuum to high pressure, including atmosphere. A high direct current (DC) voltage can generate a corona discharge. Alternatively, an alternating current (AC) voltage, which may be a sinusoidal or a pulsed voltage, may be used to generate a corona discharge. The onset voltage of corona (i.e., the Corona Inception Voltage (CIV)) may be determined by reference to the empirical Peek's law (1929). Corona discharges are more intense at higher frequencies of AC voltage. Corona discharges occur at high electric field intensities, which are lower than the dielectric strength of the medium. Thus, corona discharges may be characterized as a high voltage, low current, and low power discharge with a low intensity photoemission. Typically, corona discharges dissipate at most a few watt of power and often only a few milliwatts or less.
Applying a voltage across two electrodes induces corona discharges. If one of the electrodes is made with a lower radius of curvature compared to the other, a unipolar corona is generated, as in this case the corona discharge is almost entirely concentrated around the electrode with the higher curvature, i.e. the discharge electrode. In the corona discharge, a plasma is created around the discharge electrode. For a point-to-plane corona, the electrode is usually a metal needle made of material such as stainless steel. The tip of the needle is sharpened to a cone shape having a tip radius of about a few to a few hundreds micrometers, and the plane electrode separated from the tip by a distance of a few to a few tens of mm. The plasma usually exists in a region of the gas extending about 0.5 mm away from the metal needle point-tip. In the unipolar region outside this plasma region, charged species diffuse toward the plane electrode.
Corona discharges have commercial and industrial applications. In particular, corona discharges have been used as ionization sources in Mass Spectrometer (MS) and Ion Mobility Spectrometer (IMS) applications that are used to detect the chemical species in the gas phase. Mass spectrometers are analytical instruments that are designed for measuring mass-to-charge ratio (m/z) of gas-phase ions in a vacuum chamber. Specific types of mass spectrometers include, for example, quadrupole mass filter, quadrupole ion trap mass spectrometer (QIT) and linear ion trap mass spectrometers. These spectrometers utilize the stability or instability of ion trajectories in a dynamic electric field to separate ion according to ions' mass-to-charge ratio. Another specific type of mass spectrometer is the time-of-flight mass spectrometer (TOF). In a TOF mass spectrometer, the mass ions are repelled or pushed into a field-free flight tube, and separated and identified based on their different flight time due to their different mass-to-charge ratios. Mass spectrometers when interfaced with gas chromatograph (GC) or liquid chromatograph (LC) become a powerful analytical instrument GC-MS and LC-MC. There are many different types of ionization sources that are commonly used in mass spectrometers (e.g., electron ionization (EI) and electrospray ionization (ESI) sources). However, the use of corona discharge as an ion source is attractive due to its simplicity and the possibility of atmospheric pressure ionization.
Ion mobility spectrometers (IMS) like mass spectrometers are analytical instruments designed for gas-phase analysis and are used as detectors in gas chromatography. In these spectrometers, ions are separated at ambient pressure in according to their individual velocities as they drift through an inert gas driven by an electric field. The ionization source in conventional IMS is a radioactive nickel (63 Ni) source, which provides beta emission ionization. The radioactivity of the source necessitates complicated handling and safety procedures to avoid leaks. These limitations, together with the problems associated with licensing and waste disposal, has limited the acceptance of IMS in the market place. Like the case of mass spectrometers, the use of corona discharges in IMS is attractive due to its simplicity.
Corona discharges, which are designed for use in mass spectrometers, are described, for example, by U.S. Pat. Nos. 3,621,241, 4,144,451, 4,667,100, 4,023,398, and 5,070,240. Similarly, U.S. Pat. Nos. 6,822,225, 6,225,623, 6,407,382 and 5,684,300 describe the use of corona discharge ionization sources in IMS applications. Corona discharges also have been used in Atmospheric Pressure Chemical Ionization source (APCI) Mass Spectrometers, which measure chemical species in liquid-phase.
The corona discharge ionization sources used in MS or IMS generally have a simple point-to-plane geometry. The “point” electrode is a metal needle with a sharp point-tip. However, despite several years of development work, the corona discharge sources designed for MS or IMS instruments lack stability and reliability. Firstly, this type of point-to-plane geometry source is not an efficient ionization source for IMS and MS applications because the plasma region of the corona discharge is an extremely small volume around the point-tip, compared to the source chamber volume. The overall detection sensitivity of an IMS or MS instrument is proportional to the ionization efficiency. The lower ionization efficiency results in the lower detection sensitivities. Further, any defect of the needle point-tip (e.g. heating of an unexpected spark or long-term electrochemical effects to the electrode material) increases the point-tip radius, which makes the corona unstable and degrades ion-mass detection capability. Thus, conventional corona discharge ionization sources do not provide robust ionization sources for MS and IMS applications.
Consideration is now being given to new designs of corona discharge ionization sources for mass spectrometer, ion mobility spectrometer and other applications. Attention is paid to improving the performance characteristics including the reliability and stability of the corona discharge ionization sources. In particular, attention is directed to the point-to-plane geometry and to the configuration of the electrodes that are used to generate the corona discharge for mass spectrometer and ion mobility spectrometer applications.